Shown in FIG. 1 a schematic drawing of a conventional laser irradiation device of a laser printer including a laser diode exciter 2 exciting laser diode 1 to emit laser beam, a cylindrical lens 3 changing the diverging laser beam from the laser diode 1 into parallel laser beam, a polygonal mirror 4 changing the direction of irradiation of the incident parallel laser beam as it turns according to the lapse of time, a motor 5 driving the polygonal mirror 4, f-.theta. lens 6 directing the laser beam reflected at the polygonal mirror 4 to a planar surface along a irradiation direction S, a reflection mirror 7 positioned at the forward end of the irradiation surface to identify the starting points of each surfaces of the polygonal mirror 4 and a irradiation starting signal generator 8 generating irradiation starting signals in response to the laser beam reflected at the reflection mirror 7 and sensed by a light sensing element SS.
Shown in FIG. 2 is a controller controlling the excitation of the laser diode 1 including a reset signal generator 13 generating a reset signal to a counter 15 in response to the irradiation starting signal generated in the irradiation starting signal generator 8, an off signal generator 14 generating an off signal to turn-off the laser diode 1 when the irradiation starting signal is applied, a counter 15 counting the oscillation of a clock oscillator 16 for a preset number, an on signal generator 17 generating an on signal for turning-on the laser diode 1 in response to the output of the counter 15 and a signal synthesizer 18 receiving and synthesizing the on signal of the on signal generator 17, the off signal of the off signal generator 14 and a picture signal, and transmitting the synthesized signal to the laser diode exciter 2.
The laser beam generated in the laser diode 1 is directed to the polygonal mirror 4 functioning as a deflector after having changed to a parallel laser beam through the cylindrical lens 3. The polygonal mirror 4, rotating in the direction shown by the arrow S by the motor 5, changes the reflection angle according to the lapse of time repeatedly, that is, changes angular velocity to linear velocity continuously. The parallel laser beam reflected at the polygonal mirror 4 irradiate along the direction of irradiation S in a constant speed through the f-.theta. lens 6 for irradiating on a planar surface.
A reflection mirror 7 is positioned at the forward end of a irradiating surface for indicating the position of the laser beam as each mirror surfaces of the polygonal mirror 4 turns and the irradiation starting signal generator 8 is positioned at a fixed position in the path of the reflected laser beam. A certain pulse is generated from the irradiation starting signal generator 8 on the moment when the laser beam passes thereover, which is a signal based on which the picture signal is generated.
Herein, the laser diode 1 should be controlled to generate the irradiation starting signal on the time when the laser beam passes over the reflection mirror 7, a circuit for which is a circuit as shown in FIG. 2.
The control operation of FIG. 2 is explained referring FIG. 3. On irradiation signal input to the reset signal generator 13 and the off signal generator 14, the reset signal generator 13 generates a reset signal and transmits the reset signal to the counter 15, and the off signal generator 14 generates an off signal shown in FIG. 3(C) which is generated on the synchronization of the irradiation starting signal with a falling edge.
Then, when the counter 15, having set a value to count on receiving a counting set signal, counts the oscillation from the clock oscillator 16 to transmit the counted value to the on signal generator 17, the on signal generator 17 transmits the on signal shown in FIG. 3(B) to the signal synthesizer 18. Thereafter, the signal synthesizer 18 generates the laser diode excitation signal as shown in FIG. 3(E) which is a synthesized signal of the picture signal, the on signal and the off signal. In this time, the irradiation starting signal, independent of the polygonal mirror 4, turned-on slightly before the time when next mirror surface coming in considering the speed of rotation of the polygonal mirror 4.
However, because the generation time of the irradiation starting signal should be set slightly before the expected time measured of the starting point of the next irradiation surface, it has been a problem to require a complicated circuit as shown in the control part of FIG. 2. Further, it has a restriction in setting the value to count to change the resolution through changing the speed of the polygonal mirror, and because the oscillation of the oscillation clock should be high frequency to rectify this restriction, it also raise a problem of developing a greater noise and disturbance of the picture in the laser printer system.